Pulp cooking with particular alkali profiles

ABSTRACT

Yield, particularly when treating hardwood chips, can be improved by at least 1-2% in a kraft cellulose pulping process by keeping the temperature and effective alkali (EA) low during impregnation, and by keeping the EA low in at least a first cook stage. After cooking, the pulp is subjected to cooling low EA liquor, e.g. to reduce its temperature to below 120° C. (preferably below 100° C.) with an EA below about 5 g/L (expressed as NaOH). Continuous treatment in a continuous digester system is preferred, with the EA below about 20 g/L during impregnation and the first cook.

BACKGROUND AND SUMMARY OF THE INVENTION

[0001] The term “chemical pulping” applies to the process of treating comminuted cellulosic fibrous material, for example, hardwood or softwood chips, with an aqueous solution of chemicals which dissolve the non-cellulose components of the material, and some of the cellulose components, to produce a slurry of cellulose fibers that can be used to produce cellulose paper products. The commercially significant chemical pulping process in the late twentieth century is the alkaline process. Two such alkaline processes are the “kraft” process and the “soda” process. In the kraft process, the active chemicals with which the wood is treated are sodium hydroxide [NaOH] and sodium sulfide [Na₂S]. The aqueous solution of hydroxide and sulfide is referred to as “kraft white liquor”. In the soda process, very little or no sodium sulfide is present.

[0002] The treatment is performed at a temperature of over 100° C., and the process is typically under superatmospheric pressure, preferably 5-10 bar. The hydroxide is a strong base and the process is performed in a highly basic, or alkaline, state, for example, at a pH typically greater than 12. As the chemistry is presently understood, the hydroxide dissolves the non-cellulose compounds of the wood which bind the cellulose fibers together, that is, the “lignin”, and the sulfide acts to protect the cellulose from degradation by the hydroxide.

[0003] The rate of reaction of the kraft and soda pulping processes is dependent upon the temperature of the reaction and the concentration of cooking chemical. The higher the temperature and the higher the chemical concentration, the more rapid the reaction of the sodium hydroxide, also known simply as “alkali”, with the wood material. The concentration of cooking chemical is typically expressed as “active alkali” (AA) or “effective alkali” (EA). In this application the term “effective alkali as equivalent NaOH” will be used exclusively to express the concentration of cooking chemical. EA is typically given by the sum of the concentration of NaOH plus one-half the concentration of Na₂S expressed in grams per liter (g/L) of equivalent NaOH, that is,

EA=[NaOH]+½[Na₂S] g/L NaOH

[0004] In earlier chemical pulping processes employing the kraft process, in either batch or continuous mode, the cooking chemical was introduced essentially in its entirety at the beginning of the treatment. As the treatment progressed the alkali concentration diminished as the cooking chemicals were consumed in the pulping reaction. For example, in what is typically referred to as a “conventional kraft cook”, the initial EA concentration to which the cellulose is exposed may be 40 g/L or higher. This initial EA then declines during the treatment such that the final EA at the completion of the cook may approach 5 g/L or lower.

[0005] In the late 1970s and early 1980s, in the pioneering work done by the Swedish research firm STFI, the benefits of “leveling out” the alkali profile throughout the cooking process by decreasing the initial EA concentration and increasing the final EA concentration was introduced. Johanson, Mjoberg, Sandstrom and Teder (Svensk Papperstidning, 87(10):30 (1984)), in discussing this process, calculate an EA concentration, in a continuous digester employing counter-current treatment, of between 10-15 g/L initially and 5-10 g/L at the end of the cook. The EA concentrations rise and fall during the treatment by introducing white liquor and extracting spent cooking chemical, known as “kraft black liquor”. This process of “split white liquor addition” and counter-current treatment, known as “modified kraft cooking”, was adopted broadly throughout the pulping industry in the 1980s. For example, the process and associated equipment were sold under the trademark MCC by Ahlstrom Machinery Inc., of Glens Falls, N.Y. Later, the counter-current process was extended even further by the addition of white liquor to the counter-current wash zone, known as the Hi-Heat wash zone, in a process marketed by Ahlstrom Machinery under the trademark EMCC.

[0006] In the 1990s, Marcoccia, et al. introduced the Lo-Solids® cooking process and equipment which provided the next dramatic improvement to the kraft cooking process. See U.S. Pat. Nos. 5,489,363; 5,536,366; 5,547,012; 5,575,890; 5,620,562; and 5,662,775. Marcoccia, et al. recognized that the concentration of dissolved reaction products, including dissolved lignin, dissolved cellulose, and dissolved hemicellulose, among other dissolved compounds, were detrimental not only to the latter stage, or “residual delignification” stage, of the pulping process, as proposed by Johanson, et al., but also to the principal stage of delignification, that is, the stage known as the “bulk delignification” stage. By the selective removal of spent cooking liquors and replacement with cooking chemical and dilution liquors, for example washer filtrate having a lower concentration of dissolved materials, early or at the beginning of the pulping process a stronger, cleaner cellulose pulp could be produced.

[0007] In co-pending U.S. patent application Ser. No. 08/911,366 filed on Aug. 7, 1990 (Attorney Docket 10-1216), the benefits of treatment of the cellulose material in a kraft cook at lower cooking temperature is disclosed. This process is especially effective when used in conjunction with the Lo-Solids® cooking process and equipment described in the above-referenced patents.

[0008] In all chemical treatment of wood to produce a cellulose pulp, the cellulose and non-cellulose constituents are not segregated in the wood but are typically intermingled with each other. It is difficult to dissolve the undesirable non-cellulose constituents without dissolving some of the desirable cellulose. As a result, in the chemical treatment of wood, though the original wood may typically consist of 60 to 70% desirable cellulose (and hemicellulose), typically only about 50 % of the usable cellulose is retained in the final product. (It is to be understood that the product of the pulping process typically also contains other tolerable, non-cellulose constituents of the wood, such as some residual lignin.) Some of the desirable cellulose is dissolved at the same time as the undesirable non-cellulose. This percentage, by weight, of the amount of cellulose retained compared to the amount of wood introduced to the process is referred to as the “yield” of the process. Note that a 1% increase in yield for a typical 1000 ton-per-day pulp mill, which sells pulp at approximately $500.00 per ton, can mean over a million dollars in revenue per year. Thus, single-digit increases in yield can have significant impact upon the profitability of a pulp mill.

[0009] In a paper entitled “Improved Pulp Yield by Optimized Alkaline Profiles in Kraft Delignification” (Presented at the TAPPI symposium “Breaking the Pulp Yield Barrier” on Feb. 17-18, 1998), the inventors, and others under the direction of the inventors, showed that a low and uniform alkali profile and a low temperature profile in a kraft cook of birch chips improves the cellulose and hemicellulose yield, as pursuant to the invention. (Specifically, the yield of the hemicellulose “xylan” is improved.) This publication discusses laboratory experiments showing the theoretical basic aspects of the invention, rather than how the preferred alkali and temperature profiles can be effected in a commercial pulp mill.

[0010] The present invention comprises or consists of methods and apparatus for effecting the desired low and uniform alkali treatment that has been found desirable according to the present invention. One embodiment of this invention comprises or consists of a method of treating comminuted cellulosic fibrous material to produce cellulose pulp, comprising: (a) Treating the material with a first alkaline liquid having a first effective alkali concentration and at a temperature less than 120° C. (b) Treating the material with a second alkaline liquid having a second effective alkali concentration while heating the material to a temperature above 120° C. (c) Treating the material with a third alkaline liquid having a third effective alkali concentration at a temperature greater than 140° C. to delignify the material. And (d) treating the material with a cooling liquid to cool the material to a temperature less than 120° C.; wherein the first, second, third initial effective alkali concentrations are all less than 30 g/L, typically less than 25 g/L, preferably less than 20 g/L as NaOH, and the EA concentration in (b) and (c) is less than 25 g/L, preferably less than 20 g/L. The cooling liquid typically has an EA of from 0-5 g/L so that the EA gradually decreases to a level below 5 g/L as the temperature is gradually reduced; and the temperature of the cooling liquid is typically below 110° C., e.g. below 90° C.

[0011] The method may also further include (e), between (c) and (d) of treating the material with a fourth alkaline liquid at a temperature greater than 140° C., the fourth alkaline liquid having a fourth effective alkali concentration. This fourth initial EA concentration may be less than 30 g/L, for example, less than 25 g/L or less than 20 g/L (e.g. about 15 g/L) as NaOH, but according to the invention the fourth EA concentration need not be limited to this lower EA concentration. The fourth initial EA concentration may also be greater than 30 g/L. The method may also further include (f, between steps (e) and (d), of treating the material with a fifth alkaline liquid at a temperature greater than 140° C., the fifth alkaline liquid having a fifth initial EA concentration. The fifth initial EA concentration may be less than 30 g/L, for example, less than 25 g/L or less than 20 g/L as NaOH, but according to this invention the fifth EA concentration need not be limited to this lower EA concentration. The fifth initial EA concentration may also be greater than 30 g/L.

[0012] The desired EA concentrations are preferably achieved by introducing dilution liquid to the alkaline liquids prior to contacting it with the material, in particular, at least introducing dilution liquor to the second alkaline liquor of (b). This dilution liquor preferably consists of or comprises washer filtrate, evaporator or heat exchanger condensate, spent cooking liquor, fresh water, or combinations thereof. It will be understood by those skilled in the art that the introduction of dilution to the cooking chemical may also be effected during the preparation, storage or transfer of the cooking chemical. For example, it is within the scope of this invention to reduce the alkali concentration of the cooking chemical introduced to the material by diluting the cooking chemical during the recausticization process, or in any other phase of the liquor preparation process.

[0013] The method of the invention preferably produces a cellulose pulp having increased yield compared with conventional methods (e.g. (a) with an initial EA over 30 g/L and a temperature over 120° C., and (b) or (c) with an initial EA over 25 g/L), for example, yield increases of at least 1%, and preferably at least about 2% can be produced. This is particularly true of the application of this process to the treatment of hardwood chips, for example, birch chips.

[0014] In the method described above, the second and fourth and fifth alkaline liquors may be obtained by adding to cooking liquor heated dilution liquor having a low or substantially zero alkali concentration. Preferably (a) is practiced so that the EA gradually decreases while the temperature remains substantially the same; and preferably (a)-(d). or (a)-(f), are practiced continuously, in fact even in the same upright vessel, though these steps may also be practiced in more than one vessel. Though this specification will almost exclusively discuss the implementation of the present invention for continuous treatment, it would be understood by one skilled in the art that the present invention can also be implemented in a non-continuous or “batch” process.

[0015] According to another aspect of the invention a method of continuously treating hardwood chips, using a continuous digester system where the chip slurry primarily flows downwardly during treatment, is provided. The method comprises: (a) Impregnating the hardwood chips of the slurry in a first stage using a first alkaline liquid with an initial EA at the start of the first stage of about 25 g/L or less, and at a temperature of between about 90-11° C., the EA gradually diminishing by at least 10 g/L during the first stage, and so that at the end of the first stage it is about 10 g/L or less. (b) Gradually heating the hardwood chip slurry to a cooking temperature of about 140-180° C. as the slurry continuously moves through a second stage substantially contiguous with the first stage, by treating the slurry with a second alkaline liquid, the EA of the slurry starting at the beginning of the second stage at less than 15 g/L, and increasing at least about 5 g/L during the second stage, but not exceeding about 25 g/L. (c) Cooking the hardwood chip slurry in a third stage, using a third alkaline liquid, at a temperature that remains substantially constant and is between 140-180° C. and at an initial EA at the start of the third stage of below 25 g/L, and gradually decreasing by at least about 5 g/L and so that the EA at the end of the third stage is below 20 g/L. (e) Optionally subjecting the hardwood chips to at least a second cooking in a fourth stage at approximately the same substantially constant temperature in the third stage, using a fourth alkaline liquid. (d) And in a last stage, using a last alkaline liquid, gradually cooling the hardwood chips slurry to a temperature less than about 110° C. and so as to reduce the EA of the slurry at least about 5 g/L from the beginning to the end of the last stage, and so that the slurry has a final EA of less than about 5 g/L; wherein (a)-(d) are practiced so as to increase yield of pulp produced by at least 2% compared to practicing (a) at a temperature of greater than about 120° C. and an initial EA of over 30 g/L, and practicing (b) or (c) at an initial EA of over about 25 g/L.

[0016] In the above method, (e) is practiced, so that the EA increases from the beginning to the end of the fourth stage by at least 5 g/L. Also, (a) may be practiced at least in part by extracting liquor from the slurry at approximately the interface between the first and second stages; and wherein (b) may be practiced at least in part by adding a combination of heated cooking and dilution liquor at approximately the interface between the second and third stages so that the heated liquor flows substantially countercurrent to the chips slurry; and (c) may be practiced at least in part by extracting liquor at approximately the interface between the third and fourth stages; and (e) may be practiced at least in part by adding a combination of heated cooking and dilution liquid below the extraction at approximately the end of the fourth stage, to flow substantially countercurrent to the hardwood chips slurry; and (d) may be practiced at least in part by introducing dilution liquor at a temperature below about 110° C. adjacent a discharge of pulp from the digester system.

[0017] According to another aspect of the present invention there is provided a continuous digester system comprising: At least one substantially upright digester vessel having first, second, third, fourth, and last consecutive stages, each stage substantially contiguous with the previous stage and having an interface therewith. An inlet adjacent the top of the first stage. A first liquor extraction device at approximately the interface between the first and second stages, including an extraction screen. A first withdrawal and recirculating system at approximately the interface between the second and third stages, including a first recirculation screen, pump, heater, at least one cooking and dilution liquor addition conduit, and a recirculation pipe at approximately the level of the first recirculation screen. A second extraction screen at approximately the interface between the third and fourth stages. A second withdrawal and recirculating system at approximately the interface between the fourth and next consecutive stage, including a second recirculation screen, pump, heater, at least one cooking and dilution liquor addition conduit, and a second recirculation pipe at approximately the level of the second recirculation screen. Cooling liquor introducing devices adjacent the bottom of the digester vessel, at the bottom of the last stage. And a pulp discharge from the last stage, adjacent the bottom of the digester vessel.

[0018] In the digester system the second recirculation screen may be substantially at the interface between the fourth and last stages, or a fifth stage (with associated recirculation system as described with respect to the above systems) provided at approximately the interface between the fourth and fifth stages. All of the first through last stages may be in a single upright vessel (i.e. the at least one vessel consisting essentially of one vessel). The first through last stages may also be performed in more than one vessel. For example, the first stage or the first and second stages may be performed in a first vessel, for example, in a pretreatment or impregnation vessel, and the rest of the stages performed in a second vessel, or digester.

[0019] It is the primary object of the present invention to provide increased yield in the kraft pulping of cellulose, including hardwood chips. This and other objects will become clear from a detailed description of the invention, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a simple block diagram illustrating one exemplary form of a method of the present invention;

[0021]FIG. 2 is a block diagram illustrating a laboratory technique that may be used to evaluate a method of the present invention;

[0022]FIG. 3 is a graph that displays the alkali and temperature profiles of the laboratory trials of a method shown in FIG. 2.

[0023]FIGS. 4 and 5 are graphs that display the yield results of the trials shown in FIGS. 2 and 3;

[0024]FIG. 6 is a graph which displays another set of alkali and temperature profiles for laboratory trials of a method according to the present invention;

[0025]FIG. 7 is a graph that displays the yield results of different components of the pulp produced by using the profiles shown in FIG. 5;

[0026]FIG. 8 shows an exemplary apparatus for practicing a method of the present invention, along with graphical displays of representative alkali and temperature profiles at various locations in the apparatus; and

[0027]FIG. 9 schematically shows a continuous digester, and an actual alkali profile of the continuous digester operating according to a method of the present invention, compared to conventional operation.

DETAILED DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 is a schematic illustration 10 of the method of the present invention. The method comprises or consists of a series of treatments of a slurry of comminuted cellulosic fibrous material, for example, hardwood chips 11. In the first stage 12, the slurry is treated with a first alkaline liquid 18 at a temperature less than 120° C. The initial alkalinity of the liquid 18, expressed as effective alkali (EA) is typically less than 30 g/L as NaOH, for example, less than 25 g/L, preferably, between 15 and 25 g/L (or any narrower range within this broad range, e.g. 18-22 g/L). This lower EA is preferably achieved by introducing low-EA-containing dilution liquid to stage 12, for example, by adding dilution liquid to the alkaline liquid introduced at 18 or by adding dilution liquid directly to the slurry by a conduit 18 a. Dilution liquid may comprise or consist of washer filtrate, evaporator or heat exchanger condensate, weak black liquor, fresh water, or combinations thereof. The dilution of the cooking liquor may also be effected during preparation, storage or transfer of the cooking liquor. The temperature in stage 12 is typically between 80 and 120° C., preferably, between about 90 and 11° C.

[0029] After treatment at 12, the slurry passes to a second treatment stage 13 in which the slurry is treated with a second alkaline liquid 19 while the slurry is heated to a temperature greater than 120° C. Again, the liquid 19 typically has an EA concentration of less than 30 g/L as NaOH, for example, less than 25 g/L, preferably, between 15 and 25 g/L (or any narrower range within this broad range, e.g. 18-22 g/L). Again, this lower EA is preferably achieved by introducing low-EA-containing dilution liquid to stage 13, for example, by adding dilution liquid to the alkaline liquid introduced at 19 or adding dilution liquid directly to the slurry by a conduit 19. Again, the dilution liquid may comprise or consist of washer filtrate, evaporator or heat exchanger condensate, weak black liquor, fresh water, or combinations thereof. The heating that occurs during stage 13 is typically achieved by circulating heated liquor through the slurry. The temperature of the slurry is typically raised to a temperature approaching typical kraft or soda cooking temperatures, for example, to a temperature of at least 140° C., typically between 140-180° C., preferably, 140-160° C.

[0030] Following the treatment and heating stage 13, the slurry is then treated with a third alkaline liquid 20 while the temperature of the slurry is maintained at the temperature above 140° C., again, typically between 140-180° C., preferably, 140-160° C. The initial EA of the treatment liquid 20 is again kept to a relatively low concentration of less than 30 g/L as NaOH, for example, less than 25 g/L, preferably, between 15 and 25 g/L (or any narrower range within this broad range, e.g. 18-22 g/L). This lower EA is preferably achieved by introducing low-EA-containing dilution liquid to stage 14, for example, by adding dilution liquid (e.g. the types described below) to the alkaline liquid introduced at 20 or adding dilution liquid directly to the slurry by a conduit 20 a. During stage 14, typically referred to as the “bulk delignification” stage, the principal delignification reaction takes place.

[0031] Stage 14 may be followed by a further delignification stage 15 in which the slurry is treated with alkaline liquid 21. Unlike the earlier stages, the liquid 21 introduced to stage 15 may contain a broad range of EA concentrations. For example, a low EA in stage 15, for example, an initial EA of less than 30 g/L as NaOH, or less than 25 g/L, or preferably, between 15 and 25 g/L, can result in the decreased dissolution of hemicellulose and result in a pulp having greater hemicellulose content. On the other hand, a higher initial EA in stage 15 can result in more hemicellulose dissolution and less hemicellulose in the resulting pulp. Since the content of hemicellulose in the pulp affects the properties of the resulting paper, the EA of stage 15 can be varied to produce the desired properties in the resulting pulp. Stage 15 is typically maintained at a temperature above 140° C., typically between 140-180° C., preferably, 140-160° C. More than one treatment stage 15 may follow stage 14. In addition, stage 15 may be omitted, as indicated by the dotted lines, and stage 14 may be followed immediately by stage 16.

[0032] Stage 16 is a cooling stage in which the treatment is terminated by introducing cooler liquid 22, typically having a much lower EA concentration, to the slurry discharged from stage 14 or 15. Typically, liquid 22 is introduced to cool the slurry to a temperature less than 120° C., typically less than 100° C. Cooling liquid 22 is typically washer filtrate, evaporator or heat exchanger condensate, weak black liquor, fresh water, or combinations thereof. The liquid 22 typically has an EA less than 10 g/L, for example, between 0 and 5 g/L as NaOH (or any narrower range within this broad range). The slurry 17 discharged from stage 16 typically comprises or consists of delignified chips or pulp having little or no EA concentration and a temperature less than 100° C. Typically, slurry 17 is forwarded on to further processing, such as brown stock washing for chemical recovery and bleaching, if desired.

[0033] The method described with respect to FIG. 1 may be effected in conventional cooking devices, including in a batch digester or a continuous digester. One preferred apparatus for effecting continuous treatment is described below. The application to a batch process is effected by varying the concentration of cooking liquors and temperature via the liquor circulation common to conventional batch digesters.

[0034]FIG. 2 illustrates a schematic diagram of a laboratory procedure used to evaluate the process disclosed in FIG. 1. Stages 32, 33, 34 and 35 correspond to stages 12, 13, 14, and 15, respectively, of FIG. 1. The cooks were carried out in a university laboratory as described in the Achren, et al. article referenced above. Specifically, the hardwood material used was fresh birch chipped and screened at a commercial pulp mill. Before cooking, the chips were screened for the thickness fraction 2-6 mm to be used in the study. All visually observed pieces or barks and knots were removed prior to pulping.

[0035] The cooks were divided into four stages, i.e., two impregnation stages, and two cooking stages. Three EA profiles of A)-C) were applied in the impregnation stages and the first cooking stage. Initial EA concentration was adjusted within the range of 4-32 g/L (NaOH) in the cooking stage 2 for all three EA profiles. Three cooks were prepared from each combination of EA profiles with a final target kappa number of 24, 18 and 14. EA charges and EA concentrations used are summarized below. TABLE 1 Impregnation Impregnation Cooking stage Cooking stage 1 (32) 2 (33) 1 (34) stage 2 (35) Pro- EA charge EA charge EA charge Initial EA file on wood % on wood % on wood % concentration, g/L A 10 7 3 5, 8, 12, 18, 25, 32 B 10 7 0 4, 8, 11, 18, 25 C 10 4 3 5, 8, 11, 18, 24

[0036] In the above Table, EA represented as a charge on wood is a weight percent of the chemical charged per weight of the wood treated or pulp produced. Note that the EA of the liquid with which the chips were treated in the tests indicated by FIG. 2 is shown by the curves of FIG. 3. In this case, a charge of 10% EA on wood corresponds to an EA of about 22-23 g/L as NaOH, as indicated by the initial peak of FIG. 3.

[0037] The profile in Table 1 most representative of the invention is Profile C; however, Profiles A and B are not prior art, merely less representative of the results achieved according to the invention.

[0038] The white liquors used in the laboratory cooks were artificially prepared from technical grade chemicals (NaOH and Na₂S). The white liquor used in impregnation step 1 (32) contained 100 g/L. (as NaOH) and its sulphidity was 50%. The same EA but a lower sulphidity (35%) was used in impregnation step 2 (33) and cooking stage 2(35 ) as the white liquor used in cooking stage 1 (34) contained 200 g/L and had a 35% sulphidity. The birch black liquor (13 g/L) used in the first and second impregnation stages was from the commercial pulp mill. The black liquor used in cooking stage 2 (35) was obtained from cooking stage 1 (34). The digester used for the steaming as well as for the two-stage impregnation and cooking stage 1 (34) was a forced circulation unit with a volume of 25-L. the wood charge was 400 g OD chips. Cooking stage 2 (35) was carried out in 1-L autoclaves which were kept rotating in an oil bath. The pulp charge was 100 g OD obtained from cooking stage 1.

[0039] The birch chips were steamed for 20 min. at 100° C. and at atmospheric pressure. The steamed chips were then pretreated in impregnation step (32) with a preheated mixture of black and white liquor. The impregnation liquor was forced into the digester with the aid of 4 bar N₂ pressure. The impregnation step 1 (32) lasted for 60 min. at 95° C. and the liquor-to-wood ratio was 4.5:1. At the end of impregnation step 1 (32) the spent liquor was drained off and analyzed. The spent liquor contained 5-6 g/L.

[0040] In the impregnation step 2 (33) the liquor consisted of mill black liquor and water (a:1 v/v), with the appropriate amount of white liquor to give the desired EA charge. The impregnation liquor was forced into the digester with the aid of 4 bar N₂ pressure. The impregnation time in this step (32) was about 40 min. including the time it took to bring the digester from 95° C. to the cooking temperature 153° C. At the cooking temperature a liquor sample was taken and cooking stage 1(34) began by forcing white liquor into the digester with the aid of 10 bar N₂ pressure. The liquor-to-wood ratio was 4.6:1 for A) and C)-profiles and 4.5:1 for B). the chips were then delignified for 60 min. for A) and B) and 80 min. for C), all at 153° C. After cooking stage 1 (34) the liquor was drained off, analyzed, and saved for use in the subsequent cooking stage 2 (35). The pulp was defibrated in a Wennberg disintegrator with a minimum amount of water, centrifuged and homogenized.

[0041] After impregnation step 2 (33) the spent liquor contained about 10 g/L EA (expressed as NaOH) from both profile A) and B), and approximately 4 g/L from profile C). After cooking stage 1(34) the spent liquor from profile A) contained about 8 g/L, from profile B) about 4 g/L and from profile C) about 2 g/L. The kappa number after cooking stage 1 (33) was 44.9 for profile A), 57.0 for profile B) and 55.3 for profile C).

[0042] The pulps cooked according to profiles A), B) and C) were divided into portions of 100 g (OD) and delignification was continued in cooking stage 2 (35) in a 1-L autoclave. The cooking liquor comprised 0.6 L black liquor from the respective cooking stage 1(34) and of varying amounts of water and white liquor to give the desired EA concentration. The consistency in cooking stage 2 (35) was 10% and cooking temperature was either 153° C. or 165° C. and the cooking time varied to reach the desired kappa level. At the end of a cook, the autoclave was cooled for 20 min. in tap water. Spent liquor was drained and analyzed. The pulp was rinsed in water, centrifuged and put in a plastic jar with 2 L of water. After 16 hours of soaking, the pulp was defibrated in a 2-L disintegrator, washed and screened with a Sommerville screen, centrifuged, homogenized and weighed. The reject from the Somerville screen was dried at 105° C. and weighed.

[0043] A hexeneuronic acid (HexA) content analysis was carried out after application of the acid hydrolysis of the pulp (100° C., 4 hours at pH 3). The kappa number was determined before and after the hydrolysis, and the HexA content was calculated by considering that 1 kappa unit corresponds to 10 meq/kg HexA in the pulp.

[0044] Selected pulp samples from the C) profile were bleached by an ECF-bleaching sequence (D_(o)-E-D) in plastic bags immersed into a warm water bath. During bleaching the pulp was mixed manually. The pulp consistency was 10% in all stages. The retention time was 60 min., 90 and 180 min. respectively. Conditions used in the ECF sequence (D_(o)-E-D) are given below. Stage Conditions D_(o) Retention time 60 min. at 60° C. ClO₂ dose (act. Cl) 3.6% end pH 2.2-2.3 E Retention time 90 min. at 60° C. NaOH dose 1-2.5% end pH 10.9-11.7 D Retention time 180 min. at 80° C. ClO₂ dose (act. Cl) 3% end pH 2.4-2.8

[0045] Analysis methods used are summarized below. Dry solids content of chips SCAN CM:39:88 Dry solids content of pulp SCAN-C 3:78 Effective alkali in cooking liquors SCAN-N 33:94 Kappa number SCAN-C 1:77 ISO brightness (pulp) SCAN-C 11:75 Viscosity of pulp SCAN-CM 15:88 Hexeneuronic acid (HexA) content according to Vuorinen et al. /14/ Carbohydrate content of pulp according to Laver et al. /16/

[0046] Table 2 on the next page contains the cooking conditions and results for the trials described with respect to FIG. 2. Table 3, two pages hence, contains the carbohydrate composition of the pulps produced in the trials described with respect to FIG. 2. The most representative results according to the invention are identified as Profile C in the first column of each of Tables 2 and 3. The data in FIG. 4 is limited to cooks at 153° C. and the yield data has been corrected by calculating the equivalent yield for a kappa number 18. That is, data in Table 2 having higher and lower kappa numbers compared to kappa 18 was adjusted by 0.2% yield per kappa unit. TABLE 2 Cooking results and key pulping conditions for cooking stage 2 (35) Cooking Cooking Initial Residual Kappa Total yield Rejects Viscosity Brightness HexA Profile temp. ° C. time, min EA g/L EA g/L no % on wood % on wood mL/g ISO % meq/kg H-factor A 153 240 5,0 1,2 18,6 51,1 0,7 1349 33,5 67 872 A 153 120 8,4 4,4 21,1 50,7 1,1 1343 32,5 — 436 A 153 180 8,4 4,0 18,1 50,9 0,6 1317 34,8 68 654 A 153 150 11,8 7,1 17,9 — — 1329 35,0 69 545 A 153 135 17,6 12,2 16,2 49,3 0,8 1341 39,7 64 491 A 153 120 24,2 19,0 15,6 47,7 0,6 1347 42,4 — 436 A 153 80 32,4 27,4 21,9 47,7 1,2 1439 36,7 — 291 B 153 360 4,6 0,3 20,0 53,0 0,3 1421 27,1 — 1308 B 153 500 4,6 0,0 18,9 52,9 0,2 1391 26,4 70 1817 B 153 200 7,9 3,6 21,5 53,8 0,4 1461 30,3 — 727 B 153 280 7,9 3,0 19,2 53,4 0,1 1403 31,1 — 1018 B 153 460 7,9 1,8 17,6 51,8 0,5 1309 29,6 77 1672 B 153 150 11,2 6,7 22.4 53,0 0,8 1420 29,6 — 545 B 153 290 11,2 4,9 17,6 51,7 0,1 1316 30,7 75 1054 B 153 460 11.2 4,0 17,1 50,2 0,3 1251 29,5 — 1672 B 153 180 17,6 11,2 17,2 51,6 0,2 1357 33,0 — 654 B 153 300 17,6 9,8 16,5 50,2 0,0 1226 32,1 73 1090 B 153 140 24,3 18,0 18,0 50,0 0,3 1409 32,9 — 509 B 153 200 24,3 16,8 16,2 49,0 0,1 1306 34,2 — 727 B 165 130 7,6 3,1 21,6 52,6 0,6 1404 29,5 — 1331 B 165 305 7,6 1,2 17,5 50,6 0,2 1293 29,9 73 3122 B 165 260 17,6 8,3 15,2 48,5 0,1 1082 31,9 50 2662 B 165 95 24,3 18,3 18,5 49,8 0,7 1380 33,6 61 973 C 153 340 4,2 0,8 18,9 54,8 0,3 1506 26,3 — 1236 C 153 540 4,2 0,3 17,6 54,4 0,5 1412 26,4 63 1962 C 153 280 7,9 3,2 18,3 54,3 0,2 1434 28,2 — 1018 C 153 500 7,9 2,2 16,9 54,1 0,3 1343 29,6 75 1817 C 153 260 11,4 5,9 18,3 53,9 0,1 1390 29,9 — 945 C 153 460 11,4 3,6 16,4 52,6 0,7 1303 30,5 73 1672 C 153 150 17,5 11,8 20,3 53,1 0,4 1476 30,4 — 545 C 153 300 17,5 10,2 16,5 51,4 0,3 1267 31,2 74 1090 C 153 130 24,6 19,5 18,4 51,4 1,3 1475 33,1 — 472 C 153 200 24,6 17,2 16,3 50,7 0,3 1338 32,9 66 727 C 165 280 7,5 1,2 17,8 53,0 0,2 1246 28,8 — 2866 C 165 360 7,5 0,7 17,5 52,4 0,0 1222 27,9 70 3685 C 165 125 16,3 11,0 17,6 52,3 0,2 1362 34,0 — 1280 C 165 310 16,3 6,9 14,9 49,8 0,1 1006 31,3 51 3173 C 165 95 23,8 18,5 19,6 51,7 0,6 1453 34,4 — 973 C 165 240 23,8 12,4 14,1 48,6 0,0 956 32,7 41 2457

[0047] TABLE 3 Carbohydrate compositions of brown stock pulps Temp., Residual Kappa Glucan Xylan Mannan Tot. hemis Tot. sugars Lignin-free Profile ° C. g/L EA number % % % % % yield, % A 153 1,2 18,6 74,0 25,7 0,3 26,0 94,3 49,7 A 153 7,1 17,9 74,6 25,2 0,3 25,5 93,5 — B 153 1,8 17,6 74,2 25,5 0,3 25,8 93,4 50,5 B 153 18,0 18,0 76,6 23,2 0,3 23,5 93,2 48,7 C 153 0,8 18,9 74,3 25,4 0,3 25,7 91,4 53,3 C 153 3,2 18,3 74,1 25,7 0,3 25,9 92,9 52,8 C 153 5,9 18,3 74,2 25,5 0,3 25,8 94,0 52,5 C 153 10,2 16,5 75,4 24,3 0,3 24,7 95,0 50,1 C 153 19,5 18,4 77,2 22,5 0,3 22,8 94,2 50,0 C 165 1,2 17,8 75,2 24,5 0,4 24,8 93,7 51,6 C 165 11,0 17,6 75,6 24,0 0,4 24,4 94,0 51,0

[0048]FIG. 3 is a graph of representative effective alkali and temperature profiles for the three cooking trials described with respect to FIG. 2. In FIG. 3, the alkali profile A from Table 1 above is shown by graph line 40, while the profiles B and C from Table 1 are shown by graph lines 41, 42, respectively. The temperature graph for all profiles is shown by graph line 43.

[0049]FIG. 4 is a graph of the Total Brownstock Yield versus Residual EA concentration, that is the EA at the end of stage 35 of FIG. 2, for the trials described with respect to FIG. 2. In FIG. 4, the alkali profiles A-C from Table 1 are shown by graph lines 45-47, respectively; all cooks were at about 153° C. Clearly, the pulp produced by the most representative practice of the method of the present invention (graph line 47) produces a higher total yield than the less representative, referenced, methods (graph lines 45, 46).

[0050]FIG. 5 is a graph of the Total Yield as a function of kappa number for the trials described with respect to FIG. 2. In FIG. 5, the alkali profiles A-C from Table 1 are shown by graph lines 49-51, respectively; all cooks were at about 153° C. Again, the yield at kappa number for the pulps produced by the most representative practice of the method of the present invention (51) are greater than the pulps produced from the less representative, referenced, methods (49, 50).

[0051]FIG. 6 illustrates another alkali and temperature profile for a series of laboratory cooks for hardwood, according to the invention and the prior art. The data shown in FIG. 6 compare the alkali profile according to the present invention 53 to a “conventional” cook 54 and to a “High EA” cook 55. [The temperature profile for each is shown by 56.] The resulting yield data for these trials are shown in FIG. 7. The low EA profile 53 according to the present invention produces a hardwood pulp have a greater total yield, cellulose yield, and xylan yield, than either the conventional cook 54 or the high EA cook 55. In the conventional cook 54, as can be seen from FIG. 6, the initial EA concentration in the cooking liquor was about 44 g/L as NaOH, and the temperature in the representative stage leading up to cooking was over 120° C.

[0052] For the “High EA” cook 55 in FIG. 6, the initial EA concentration was 28 g/L, while for the cook 53 of the invention the initial EA concentration was about 22 g/L.

[0053]FIG. 8 illustrates a continuous digester system 100 that can be used to practice the method of the present invention and comprising apparatus according to the invention. FIG. 8 also shows the respective alkali concentration profile 101 and temperature profile 102 of the treated slurry as it passes through the vessel 105. The zones lettered A through F on the left-hand side of FIG. 8 correspond to steps (a) through (f) of the method of the present invention.

[0054] Typically, the slurry of comminuted cellulosic fibrous material 103, for example wood chips, is introduced to the top 104 of the digester vessel 105. Vessel 105 may be a single vessel or may be part of a multiple-vessel system, for example, the slurry 103 may have been treated in an initial pretreatment or impregnation vessel. For example, step A or steps A and B shown in FIG. 8 may be performed in a first vessel, for example, an impregnation vessel, and steps C-F may be performed in a second vessel, for example, a digester. The slurry 103 may be fed by a conventional feed system or preferably by a Lo-Level® Feed System, sold by Ahlstrom Machinery Inc. of Glens Falls, N.Y., as described in U.S. Pat. Nos. 5,476,572; 5,622,598; 5,635,025; 5,736,006; 5,753,075; 5,766,418; and 5,795,438. The fully-treated pulp slurry 107 is discharged from the bottom 106 of vessel 105. The slurry 107 enters the vessel at a temperature of about 80 to 120° C., for example, at about 100° C. as shown, and at an initial EA concentration of less than 20 /L as NaOH, for example, at about 18 /L as shown. Excess liquor is removed from the slurry introduced to the vessel by means of separator 108 and returned to the feed system or previous vessel via conduit 109.

[0055] While the slurry travels downward in the vessel 105 from the inlet, the temperature of the slurry is maintained at about 100° C., as shown by profile 110, while the EA concentration decreases, as shown by profile 101, as alkali reacts with the constituents of the wood. When the slurry encounters first extraction screen 111, at approximately the interface between stages A and B, some liquor is removed from the slurry and passed via conduit 112 to a Chemical Recovery System, as is conventional, or is used elsewhere as needed, for example, as a source of heat in a heat exchanger. Some of the liquor removed by screen 111 may be re-circulated by pump 113, heater 114, and conduit 115 to be reintroduced to the vessel 105 in the vicinity of the screen 111. This recirculated liquor may be augmented with additional liquors, including cooking liquors (for example, kraft white liquor, green liquor or black liquor), dilution liquids or liquids containing yield or strength enhancing additives, such as anthraquinone or polysulfide or their equivalents or derivatives, or combinations thereof. However, as shown by the dashed lines, this recirculation is not necessary to perfect the present invention. Upon reaching screen 111 the alkali content of the slurry decreases to less than 10 g/L, typically between 3-10 g/L as shown by curve 101. The treatment stage A between the top of the vessel 104 and screen assembly 111 corresponds to step (a) of the method of the present invention.

[0056] Screen assembly 111 may be a double screen assembly with one set of screens and associated liquor removal conduit located above a second screen and its associated liquor removal conduit. The upper screen assembly of screen 111 may be used to remove liquor as in conduit 112 and the lower screen assembly may be used to remove and recirculate liquor as described above with respect to structures 113, 114 and 115.

[0057] After passing screen 111, the slurry enters a heating zone or stage B between screen 111 and a first recirculating screen 116. Though the arrows 117 indicate that this heating zone is a counter-current heating zone, this zone may alternatively be a co-current heating zone. The removal of liquid via conduit 112 causes an upward flow of hot liquor 117. This hot liquid is introduced via a first circulation conduit/reintroduction pipe 121 associated with screen 116. As the slurry passes screen 116 liquor is removed and recirculated by means of pump 119, heater 120, and conduit 121. The liquor removed and circulated is augmented by cooking liquor and dilution liquor introduced via conduit 118, though cooking and dilution liquor may be introduced via separate conduits. As the slurry passes below screen 111, the counter-current flow of hot, alkali-laden liquor 117 heats the slurry as shown by curve 102 to cooking temperature, for example, to a temperature above 140° C., preferably between 140 and 160° C. The slurry is simultaneously exposed to liquor having increasing alkali content as shown by curve 122. As the alkali passes upward in stage B, it is gradually consumed such that the alkali decreases as shown by curve 122.

[0058] According to the present invention, the greatest alkali concentration introduced at the screen 116 is less than 30 g/L, preferably less than 20 /L as shown. This lower concentration is established by diluting the cooking liquor introduced by adding dilution liquor to it. The nature of the dilution liquor is as described previously and is preferably washer filtrate from a downstream washer, often referred to as “cold blow filtrate”.

[0059] The low alkali concentration below screen 111 is typically below 10 g/L, preferably between 5 and 10 g/L. This effective alkali concentration immediately below screen 111, though shown as being slightly higher than the concentrations above the screen 111, may be higher or lower than or essentially equal to the EA concentration above the screen 111. The difference shown is that of only one typical alkali concentration that may be used according to this invention. The stage B between screen 111 and screen 116 corresponds to step (b) of the method of the present invention.

[0060] The flow of liquor 117 and the introduction of steam to heater 120 is preferably controlled so that after passing screen 116, the slurry is at or near cooking temperature, that is, at at least 140° C., preferably between about 140 and 160° C. The “bulk delignification” occurs in the zone between screens 116 and 123 (a second extraction screen). As the slurry flows below screen 116, the free liquid in the slurry flows in the same direction as the flow of the cellulose material, that is, co-currently, as shown by arrows 125. Upon reaching screen 123, liquor is removed from the slurry via conduit 124 and passed to recovery or other uses. The temperature of the slurry between screens 116 and 123 is preferably maintained relatively constant, for example, at about 150° C., as shown by curve 126. The alkali concentration in this zone gradually decreases from a peak alkali at screen 116 as the alkali is consumed in the pulping reaction, as shown by curve 127. The alkali concentration is typically decreased to at least 10 g/L, preferably between 5 and 10 g/L as shown by curve 127.

[0061] The stage C between screen 116 and screen 123 corresponds to step (c) of the method of the present invention.

[0062] In one embodiment of this invention, the cooking phase is terminated at or below screen 123 by cooler dilution liquor introduced and drawn upward by the removal of liquid in conduit 124. Such is the case in older digesters, known as “cold blow” digesters where the extraction, as from screen 123, was located in the bottom of the vessel and followed by cold blow dilution. For example, such a configuration can be seen by removing zones E and F from digester 105 and following screen 123 by a cooling dilution stage or zone D. However, in a preferred embodiment of this invention, at least one additional counter-current cooking zone is present below screen 123, for example, zones E and/or F.

[0063] In the preferred embodiment, for example, in a configuration indicative of how a new digester would be built, after passing screen 123, the slurry passes into a counter-current cooking zone between screens 123 and 128 (a second recirculation screen). The removal of liquid via conduit 124 produces a counter-current flow of free liquor as shown by arrows 129. As in zone B above, the down-flowing slurry in stage E is exposed to a gradually increasing concentration of alkali as shown by curve 130. The temperature in this zone is maintained at cooking temperature, for example, at approximately 150° C., as shown by curve 131. This alkali is introduced by conduit 135 associated with screen 128. As the slurry flows downward passed screen 128, some liquor is removed and recirculated via pump 133, heater 134 and second conduit/recirculation pipe 135. This circulated liquor is preferably augmented with cooking liquor and dilution liquid via conduit 132, similar to that described with respect to conduit 118. Again, the flow of cooking liquor and dilution via conduit 132 is controlled so that the alkali concentration between screens 123 and 128 is regulated as desired to produce the most optimum carbohydrate content in the resulting pulp. Providing a higher alkali concentration, for example greater than 15 g/L, dissolves more hemicellulose so that less hemicellulose is present in the resulting pulp. Conversely, providing a lower alkali concentration in this zone, for example, less than 15 g/L, causes less hemicellulose dissolution and more hemicellulose present in the resulting pulp. Again, the cooking process can be terminated after screen 128 by introducing a cooling dilution step as shown by zone D in FIG. 8.

[0064] As discussed above with respect to screen 111, the difference in alkali concentration above and below screen 123 is representative only. According to the invention, these alkali concentration may be different or relatively the same depending upon the desired characteristics of the treatment and the species processed. The zone between screen 123 and screen 128 corresponds to step e) of the method of the present invention.

[0065] In another embodiment of the invention shown in FIG. 8, the counter-current zone E is followed immediately by counter-current zone F. In this case, the slurry passing screen 128 encounters another counter-current flow of liquor 136. Again, as described with respect to zone E above, the down-flowing slurry is exposed to a gradually increasing concentration of alkali as shown by curve 138. Also, as before, the temperature in this zone is maintained at cooking temperature, for example, at approximately 150° C., as shown by curve 139. The alkali is introduced by conduit 142 associated with screen 137. As the slurry passes screen 137, some liquor is removed and recirculated via pump 140, heater 141 and conduit 142. This circulated liquor is preferably augmented with cooking liquor and dilution liquor via conduit 143, similar to that described with respect to conduits 118 and 132. Again, the flow of cooking liquor and dilution liquor via conduit 143 is controlled so that the alkali concentration between screens 123 and 128 can be controlled as desired to produce the most optimum carbohydrate content in the resulting pulp, as described with respect to conduit 132 above. Again, the difference in alkali concentration from above screen 128 to below screen 123 is only one representation of the alkali profiles that can be achieved. These concentrations may be different or relatively the same depending upon the desired treatment and the species being treated.

[0066] Finally, below screen 137 the now essentially-fully cooked pulp encounters cooler (e.g. below 110° C. preferably below 90° C.), low-alkali-containing dilution liquor introduced via one or more conduits 144 and 145, typically by way of a conventional ring header. This cooler liquor terminates the pulping reaction, cools the pulp, and lowers its alkali concentration so that it can be discharged via conduit 107, typically with the aid of a conventional rotating discharge device (not shown in FIG. 8, schematically shown at 150 in FIG. 9). As shown by curves 146 and 147 the temperature of the pulp preferably is reduced to below 100° C., for example to 80-90° C. while the alkali concentration is reduced to less than 5 g/L, typically 0 to 4 g/L.

[0067]FIG. 9 illustrates a comparison of actual mill alkali concentration data for a continuous digester 200 operated according to the present invention, as shown by graph line 61 and a digester operated conventionally, as shown by graph line 62. The digester 200 shown in FIG. 9 is similar to the one shown in FIG. 8, though the digester in FIG. 9 only includes zones A, B, C, F and D, in that order. [In FIG. 9 structures that are the same as those in FIG. 8 are shown by the same reference numerals, the illustration in FIG. 9 being more schematic than in FIG. 8.] Compared to the alkali profile 62 in a conventional mode of operation (initial EA over 30 g/L as seen in FIG. 9), the alkali concentration profile 61 according to the present invention, particularly the initial EA, is much lower during impregnation A and heating B to cooking temperature and much higher in the final stage, or residual delignification stage F, of the cook.

[0068] While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 

What is claimed is:
 1. A method of treating comminuted cellulosic fibrous material to produce cellulose pulp, comprising: (a) treating the material with a first alkaline liquid having a first effective alkali concentration and at a temperature less than 120° C.; (b) treating the material with a second alkaline liquid having a second effective alkali concentration while heating the material to a temperature above 120° C.; (c) treating the material with a third alkaline liquid having a third effective alkali concentration at a temperature greater than 140° C. to delignify the material; and (d) treating the material with a fourth liquid to cool the material to a temperature less than 120° C.; wherein the first, second, third initial effective alkali concentrations are all less than 30 g/L, and wherein the second and third effective alkali concentrations are about 25 g/L or less.
 2. A method as recited in claim 1 wherein (a)-(c) are practiced with the first, second and third initial alkali concentrations all less than 25 g/L.
 3. A method as recited in claim 1 wherein (a)-(c) are practiced with the first, second and third initial alkali concentrations all less than 20 g/L.
 4. A method as recited in claim 1 further comprising (e), between (c) and (d) treating the material with an alkaline liquid having a fourth effective alkali concentration at a temperature greater than 140° C.
 5. A method as recited in claim 4 wherein (e) is practiced at a fourth initial alkali concentration of greater than 30 g/L.
 6. A method as recited in claim 4 wherein (e) is practiced at a fourth initial alkali concentration of less than about 15 g/L.
 7. A method as recited in claim 4 further comprising (f), between (e) and (d), treating the material at a temperature of greater than 140° C. at a fifth initial alkali concentration of less than 25 g/L.
 8. A method as recited in claim 2 wherein (a)-(d) are practiced using hardwood chips as the comminuted cellulosic fibrous material.
 9. A method as recited in claim 2 wherein the second effective alkali concentration is obtained by adding dilution liquor having a low or substantially zero alkali concentration.
 10. A method as recited in claim 3 wherein (b) is practiced to heat the material so that its temperature gradually increases, and so that the second EA concentration gradually increases, but stays below about 20 g/L.
 11. A method as recited in claim 10 wherein (c) is practiced so that the EA concentration gradually decreases while the temperature is maintained substantially constant.
 12. A method as recited in claim 11 wherein (d) is practiced with a cooling liquid having an EA of less than 5 g/L so that the EA is gradually reduced to a final level below 5 g/L while the temperature is gradually reduced.
 13. A method as recited in claim 12 wherein (a) is practiced so that the EA gradually decreases while the temperature remains substantially the same.
 14. A method as recited in claim 1 wherein (a)-(d) are practiced continuously.
 15. A method as recited in claim 7 wherein (a)-(f) are practiced continuously in the same vessel.
 16. A method as recited in claim 1 wherein (a)-(d) are practiced so as to increase yield by at least 1% compared to practicing (a) at a temperature of greater than about 120° C. and an initial EA of over 30 g/L, and practicing (b) or (c) at an initial EA of over 25 g/L.
 17. A method as recited in claim 3 wherein (a)-(d) are practiced using hardwood chips as the cellulosic material, and so as to increase yield by at least 2% compared to practicing (a) at a temperature of greater than about 120° C. and an initial EA of over 30 g/L, and practicing (b) or (c) at an initial EA of over 25 g/L.
 18. A method of continuously treating hardwood chips to produce kraft pulp, using a continuous digester system in which a hardwood chip slurry primarily flows downwardly during treatment, comprising: (a) impregnating the hardwood chips of the slurry in a first stage using a first alkaline liquid with an initial EA at the start of the first stage of about 25 g/L or less, and at a temperature of between about 90-110° C., the EA gradually diminishing by at least 10 g/L during the first stage, and so that at the end of the first stage the EA is about 10 g/L or less; (b) gradually heating the hardwood chip slurry to a cooking temperature of about 140-180° C. as the slurry continuously moves through a second stage substantially contiguous with the first stage, by treating the slurry with a second alkaline liquid, the EA of the slurry starting at the beginning of the second stage at less than 15 g/L, and increasing at least about 5 g/L during the second stage, but not exceeding about 25 g/L; (c) cooking the hardwood chip slurry in a third stage, using a third alkaline liquid, at a temperature that remains substantially constant and is between 140-180° C. and at an initial EA at the start of the third stage of below 25 g/L, and gradually decreasing by at least about 5 g/L and so that the EA at the end of the third stage is below 20 /L; (e) optionally subjecting the hardwood chips to at least a second cooking in a fourth stage at approximately the same substantially constant temperature in the third stage, using a fourth alkaline liquid; and (d) in a last stage, using a last alkaline liquid, gradually cooling the hardwood chips slurry to a temperature less than about 110° C. and so as to reduce the EA of the slurry at least about 5 g/L from the beginning to the end of the last stage, and so that the slurry has a final EA of less than about 5 g/L; wherein (a)-(d) are practiced so as to increase yield of pulp produced by at least 2% compared to practicing (a) at a temperature of greater than about 120° C. and an initial EA of over 30 g/L, and practicing (b) or (c) at an initial EA of over 25 g/L.
 19. A method as recited in claim 18 wherein (e) is practiced, and so that the EA increases from the beginning to the end of the fourth stage by at least 10 g/L.
 20. A method as recited in claim 19 wherein (a) is practiced at least in part by extracting liquor from the slurry at approximately the interface between the first and second stages; and wherein (b) is practiced at least in part by adding a combination of heated cooking and dilution liquor at approximately the interface between the second and third stages so that the heated liquor flows substantially countercurrent to the chips slurry; and wherein (c) is practiced at least in part by extracting liquor at approximately the interface between the third and fourth stages; and wherein (e) is practiced at least in part by adding a combination of heated cooking and dilution liquid below the extraction at approximately the end of the fourth stage, to flow substantially countercurrent to the hardwood chips slurry; and wherein (d) is practiced by introducing dilution liquor at a temperature below about 110° C. adjacent a discharge of pulp from the digester system.
 21. A continuous digester system comprising: at least one substantially upright digester vessel having first, second, third, fourth, and last consecutive stages, each stage substantially contiguous with the previous stage and having an interface therewith; an inlet adjacent the top of the first stage; a first liquor extraction device at approximately the interface between the first and second stages, including an extraction means; a first withdrawal and recirculating system at approximately the interface between the second and third stages, including a first recirculation screen, pump, heater, at least one cooking and dilution liquor addition conduit, and a recirculation pipe at approximately the level of said first recirculation screen; a second extraction screen at approximately the interface between the third and fourth stages; a second withdrawal and recirculating system at approximately the interface between the fourth and next consecutive stage, including a second recirculation screen, pump, heater, at least one cooking and dilution liquor addition conduit, and a second recirculation pipe at approximately the level of said second recirculation screen; cooling liquor introducing devices adjacent the bottom of the digester vessel, at the bottom of the last stage; and a pulp discharge from the last stage, adjacent the bottom of the digester vessel.
 22. A continuous digester system as recited in claim 21 wherein the second recirculation screen is substantially at the interface between the fourth and last stages.
 23. A continuous digester system as recited in claim 21 wherein all of the first through last stages are in a single upright vessel, said at least one vessel consisting essentially of a single vessel.
 24. A continuous digester system as recited in claim 21 further comprising a fifth stage between said fourth and last stages, and another recirculation system at the interface between said fourth and fifth stages. 